A typical vibrating screen comprises a frame, defined by a pair of substantially parallel side walls interconnected by transversely extending bridging members, upon which is mounted a polyurethane screen having small openings or slots for water and/or undersize particles to pass through.
The frame is mounted on a chassis via resilient mountings or linkages and the frame, and thus the screen, is typically vibrated by means of a pair of counter rotating rotors defining eccentric masses, typically driven by one or more drive motors, to impart circular or reciprocating vibratory motion to the screen.
In a grading screen the screen is arranged at a predetermined slope and material to be graded is delivered onto an upper end of the screen, typically entrained in a flow of water. The screen is vibrated at high frequency to convey the material over the screen and to cause undersize material (and water) to pass through the openings, oversize material being discharged from a lower end of the screen onto a conveyor or into a collection bay or hopper. The undersize material is typically collected in a sump and is then conveyed to a dewatering screen and/or cyclone separator for dewatering or further treatment.
It is known to provide a grading screen having a split screen, typically forming the bottom deck of a multi-deck screening apparatus, wherein the screen is divided into an upstream section having a relatively small aperture size and a downstream section having a larger aperture size, such that the screen can produce three separate products or grades of material. For example, the upstream section of the screen may have very small apertures where the 0-4 mm material can pass through and the downstream section may have larger apertures to allow the 4-6 mm material to pass through. The 6-25 mm product may then travel down the screen to pass over the downstream end of the screen onto a stockpiling conveyor. The sump of the screening apparatus may be corresponding split into two sections for receiving material from the upstream and downstream sections of the screen respectively. Typically the 0-4 mm material collected in a first section of the sump may be pumped to a cyclone separator, typically by means of a centrifugal pump, for removal of fine contaminants from the fine product, before being passed to a dewatering screen while the 4-6 mm product may be pumped directly to the dewatering screen (which may also be split into two sections to maintain the separation of the 0-4 mm and 4-6 mm products). However, the 4-6 mm product in particular tends to be very gritty and thus leads to accelerated wear of the pump used to pass such material to the dewatering screen. Different aperture sizes may be used to suit different particle sizes to provide desired grades of product (e.g. 0-2 mm, 2-8 mm and 8-25 mm).
An object of the present invention is to provide an improved screening apparatus that overcomes the problems of the prior art.